So I've read the PDF rules and have my boxed set on order (thought a preface might be appropriate for this), and was wondering how you lot are handling in-game tension between players and GM using the progress tracker for events.

Since it sounds like the tracker's out where the players can see it, when you're tracking something like reinforcements (ala day late/shilling short) or some other item that's an unknown to the party do you find this increases the game tension as if the characters feel something's coming but aren't sure what and/or does it unnerve the players to know that when they roll a C or a Bn you're moving the tracker with no explanation as to why?

Or are you finding traditional RPG tactics like ticks on paper behind a GM screen or whatnot serve you for events when the PCs wouldn't be aware of what's going on?

Yes, track out in the open and don't always tell them what it is for. You should tell them what it is for, however, in certain circumstances. For instance, I used a tracker to determine when the building they were inside would collapse from the fire ravaging its interior. You don't want it to run out without them knowing in this instance.

I keep it all hidden behind the GM screen. Is it really necessary for the players to know every time when something's about to happen? By the time the tracker reaches the end and something finally happens, then it doesn't really come as a much of a surprise to anyone, the only surprise element being WHAT it is that happens. Still, it might be fun to tease the players with this. Is it the cultists preparing a demonic rite or is it just the kettle boiling over?

It seems like another cool 'use this how it fits your needs best' tool 3e put together. You can have it out to mess with the players (or out if it's something they'd know like a chase or whatever) but it can also hide so only the GM knows the doom awaiting the players on the 7th step.

It depends. If the secrecy of the progress tracker is important to the feel of the story I'll keep it hidden, but otherwise I won't. Durring combat I have morale trackers in the open as I feel the characters will be able to notice the morale of their enemies... and I tell them that it's the morale tracker. With campaign trackers that track investigations, opposing progress etc. I usually keep them open, but don't tell the players what it's for. I also move the token on the tracker at small pauses in the game, so they know they did something or missed something but not what. They also don't know what token is theirs and what token is the opposing one. And if there is just one token it could be the Players or the NPC that I am tracking. They never know, but it gives a great feeling of "things happening" around them.

But generally I play out in the open as a GM. ALL rolls are open, because it removes the guilt from me as a GM. If you get that death blow on the dice behind the screen, then you suddenly have the choice of killing the player or not. If you roll out in the open it is no longer your descision... the dice determine fate.